During an interventional procedure a catheter may be moved to a desired location within a person under guidance of two-dimensional x-ray projection images of the person, i.e. interventional images acquired during the interventional procedure, and a pre-interventional three-dimensional image of the person, which may be a raw image or a processed image showing segmented parts of the person and which has been acquired before the interventional procedure. In order to determine the current position of the catheter, the interventional x-ray projection images showing the current position of the catheter and the pre-interventional image can be registered with respect to each other and an overlay of the registered interventional and pre-interventional images can be shown on a display. Thus, for determining the current position of the catheter, i.e. for showing the current position of the catheter within the pre-interventional three-dimensional image of the person, x-rays have to be applied to the person.